The field benefiting from this invention is that of pump-probe spectroscopy and, in a general manner, any application that requires the use of two ultra-short laser pulses separated by a known and variable time delay making it possible to scan the total time interval to be investigated.
In recent years, studies on the dynamics of biological systems have been the subject of a good deal of research. It has notably been demonstrated that such dynamics can, in certain cases, exhibit continuous evolution over several orders of time, ranging from the picosecond to the second.
Pump-probe spectroscopy is a technique that makes it possible to measure the temporal dynamics of a system during a reaction: a pump pulse triggers a reaction, then a time-shifted probe pulse measures the changes caused. It is then necessary to be able to shift one laser pulse with respect to another over the whole time range studied, with the requirement that this shift be precise to the nearest picosecond.
Currently, three means exist for generating two pulses separated by a known and variable delay.
The first means consists in using a single laser system, the beam of which is separated into two branches. One of the branches is delayed with respect to the other using a mechanical delay line. This system is limited to the picosecond-nanosecond range by the length of the delay line. Moreover, special care is required to retain adequate pointing stability during the mechanical scan.
The second means consists in using two unsynchronized laser oscillators. The delay is chosen by selecting for the amplification the pulse pair with the delay closest to the desired delay. The selection is generally performed by the opening-closing of a Pockels cell around the desired pulse. Because of the asynchronism between the two laser oscillators and the resulting jitter over time, the real delay between the two pulses thus selected is known with a precision that cannot fall below the oscillation period of the laser cavities, i.e. typically in the order of ten or so nanoseconds. In compensation, this system makes it possible to produce arbitrarily long delays.
The third means consists in using two synchronized laser oscillators, the delay of which is made to vary by varying their relative time phase. Associated with a pre-amplification selection as described in the context of the second means, this system overcomes the limitations of the previous solutions by allowing the production of long delays while retaining a precision in the order of picoseconds. However, it demands a costly and complex implementation which cannot be performed a posteriori on standard commercial laser oscillators, unless the manufacturer has made provision for this functionality right from the design of the laser. This system is therefore not accessible to most potential users. Analogously, it is possible to use laser oscillators of different frequencies by making use of the principle of stroboscopy applied to the femtosecond domain, the time coincidence between the two pulse trains being measured by an optoelectronic device. However, to work well in the proposed implementations, this approach demands that the frequencies of the two oscillators be neighboring and known with a very high degree of precision, which requires active control of the laser cavity lengths. This is generally not the case if this was not planned before the acquisition of the systems.
These various methods have been described in various publications, of which the most important are:
Bredenbeck, Helbing and Hamm., Rev. Sci. Instrum. Vol. 75 p. 4462. This article from 2004 describes a synchronous electronic scanning solution proposed by the Institut de Chimie-Physique in Zurich. Long delays are given in multiples of the repetition period of the oscillators through the selection, by a Pockels cell, of the pulse to be amplified. Short delays are generated by introducing a known phase between the two laser oscillators. With this method it is possible to attain a precision of 2 ps over an interval of up to 50 μs.
In 1999 Takagi and Adachi (RSI Vol. 70, p. 2218) introduced the method of asynchronous optical scanning. The scanning is obtained by introducing a known repetition rate difference between the two oscillators. If the introduced difference is negligible with respect to the repetition frequency f1 and f2 of the two oscillators, it has been demonstrated that this creates a magnification for the time axis f1/f2 and consequently an increment for the spectral resolution. In Takagi's study, a temporal magnification of 760000 times was measured.
In 2004 (Keimann, Goble and Holzwarth, “Time domain mid-infrared frequency-comb spectrometer”, Opt. Lett. 29, p. 1542 (2004)), this advantage was improved to ultimately obtain a temporal magnification of 45500000 times, which corresponds to 13 cm−1 of spectral resolution. These studies were conducted with femtosecond lasers, emphasizing the discrete nature of pulse spectra, which cleared the way for “frequency comb spectroscopy”, which makes use of the phenomenon of beating between each frequency line forming the spectrum of the two shifted pulses. The advantages with respect to conventional Fourier transform spectroscopy experiments lie in the sensitivity of measurement, the spectral width and the spectral resolution obtained with measurement times of a few tens of microseconds.
A method of optoelectronic synchronization has also been proposed in patent application WO/2007/045773 but the temporal resolution obtained is only beneficial when the frequencies of the two oscillators are neighboring.
It would therefore be advantageous to obtain a device for managing the laser pulses making it possible to obtain a delay between two pulses that can vary from the picosecond to the second with picosecond precision, and this while using standard femtosecond lasers.